Two-step method for dipping synthetic fiber

ABSTRACT

A two-step method for dipping a synthetic fiber includes the fiber being first dipped with an aqueous pre-dip composition followed by a dip with resorcinol-formaldehyde latex (RFL) composition, wherein the pre-dip composition includes a blocked isocyanate having at least tri-functional isocyanate groups and a latex wherein the ratio by weight of blocked isocyanate and latex is in the range of 9 to 0.7, preferably in the range of 4 to 1. A pre-dip composition for dipping a synthetic fiber includes a blocked at least tri-functional isocyanate compound and a latex, further including water, an emulsifier, and 2-vinyl pyridine copolymer and to a fiber, cord, and composite article of rubber and fiber or cord, wherein the fiber or cord has distributed over at least a surface portion thereof (1) blocked tri-functional isocyanate compound, preferably HDI-trimer; (2) an RFL adhesive; and (3) an in situ vulcanized rubber compound.

The invention relates to a two-step method for dipping a synthetic fiber wherein the fiber is first dipped with a pre-dip composition followed by a dip with resorcinol-formaldehyde latex (RFL) composition.

Fibers, such as aramid fibers, that are used to adhere to rubber compounds are commonly subjected to a dipping process for enhancing the bonding of such synthetic fibers to rubber. Such compositions may be used in manufacturing composites, for example tires wherein reinforcement fibers are bond to tire rubber compounds.

In the manufacturing of rubber composites, which include fiber reinforcement in the form of fibers, continuous filament yarns, cords, or chopped fibers, it is important to enhance the adhesion performance between the rubber matrix of the composite and the reinforced fiber materials.

Aramid fiber, for example, is a fiber used for tire reinforcement. Providing adhesion between aramid and rubber is very difficult due to the crystalline surface of the fiber. Hence an active pre-dipping process is necessary in order to get a reasonable adhesion performance between fiber and rubber. The standard method of such pre-dipping process is using an epoxy adhesive pre-dipping followed by a resorcinol formaldehyde latex (RFL) dip, which process provides good adhesion to sulfur-cured rubber for aramid reinforcement.

Thus epoxides and polyethyleneimine have been proposed as adhesion promotors for aramid fibers. Iyengar proposed a two-step adhesive system for aramid fiber adhesion to rubber. The first step or pre-dipping should consist of epoxy compound. (J. Appl. Polym. Sci., 2311, Vol. 11 (1967)). G. E. van Gills (The general Tire and Rubbers Co.), in U.S. Pat. No. 3,872,939 discloses the use of a 2% solution of glycerol diglycidyl ether as the sub-coat and top-coat which consists of 1,3-butadiene-styrene-2-vinyl pyridine copolymer along with a heat reactive 2,5-bis(2,4-dihydroxy phenyl methyl)-4-chlorophenol.

In EP 353473, apart from the conventional epoxy pre-dip compositions, also pre-dip compositions containing phenyl blocked 4,4′-diphenylmethane diisocyanate have been use. These solutions do no contain any other ingredients than water and isocyanate. We have found that aqueous blocked diisocyanate pre-dip compositions lead to fibers and cord with insufficient adhering properties to rubber.

Pre-dip compositions for polyester fibers containing tri- and higher isocyanate compounds have been described in JP 57187238. Such pre-dip compositions containing mixtures of polyfunctional isocyanates and diisocyanates have been described in JP 51037174, and containing mixtures of blocked polyisocyanates and clay (such as bentonite) have been disclosed in JP 55062269. None of these compositions, however, were very efficient in adhering rubber and none of these were commercially used.

The adhesion technology that has been developed since then is virtually always based on the use of epoxides, which is nowadays undesirable because of the ever tightening health care problems and governmental regulations. According to HSE regulations it is undesirable for human body to be exposed to epoxides. There is an increasing need to obtain safe dipping procedures, that do not make use of such toxic materials and that is substantially equivalent to epoxide-based adhesion.

To this end a two-step method for dipping a synthetic fiber wherein the fiber is first dipped with an aqueous pre-dip composition followed by a dip with resorcinol-formaldehyde latex (RFL) composition, characterized in that the pre-dip composition comprises a blocked isocyanate having at least tri-functional isocyanate groups and a latex wherein the ratio by weight of blocked isocyanate and latex is between 9 and 0.7, preferably between 4 and 1.

Fibers thus dipped maintain the adhesion properties as obtained with the conventional epoxide pre-dip, whereas the epoxide treatment as such can completely abandoned. It is therefore a preferred embodiment to perform the dipping procedure with a pre-dip composition that is substantially, or even completely, free from epoxide-containing derivatives.

In an alternative embodiment, the pre-dip composition may comprise blocked at least tri-functional isocyanate compound and a latex, which further comprises water, an emulsifier, and 2-vinyl pyridine copolymer.

The latex part of the pre-dip may be the same or different of the latex part of the RFL composition, and is a block copolymers of a conjugated olefinic block and an aromatic block. Such block copolymers are known in the art. It was found that particular good results were obtained when the latex part of the pre-dip and the RFL composition is a vinyl-pyridine block copolymer. Suitable copolymers comprise 2-vinyl pyridine, styrene, and butadiene moieties. Preferably, 10-20% of the units in the copolymer are 2-vinyl pyridine-like structures. Thus a suitable copolymer has the formula [(C₇H₇N)I(C₈H₈)_(m)(C₄H₆)_(n)]_(x), wherein the ratio in wt. % l:(m+n) is from 1:9 to 1:4 and the ratio m:n is from 2:3 to 4:1). Most preferably l:m:n=about 15:35:50.

The method according to this invention can in principle be applied to any synthetic fiber that is commonly used as reinforcement fiber in rubber compositions for tires, transport transmission belts, and the like. Such fibers preferably contain at least one of aramid, polyester, and polyester terephthalate. Particularly useful fibers within the context of the invention are fibers containing at least aramid, preferably containing PPTA and/or PPODPTA.

The method according to the invention is suitable for dipping fibers wherein the fiber is used as spun.

The blocked isocyanate has at least tri-functional isocyanate groups. Examples of such isocyanates are 1,6-hexamethylene diisocyanate (HDI) trimer, etc. Blocking groups are known in the art and are selected from ketoxime groups such as methyl ethyl ketoxime, pyrazole derivatives such as 3,5-dimethylpyrazole, esters such as malonic acid esters, caprolactam, and alkylated phenol. Most preferred blocked isocyanate is a blocked HDI-trimer, such as methyl ethyl ketoxime or 3,5-dimethylpyrazole blocked hexamethylene diisocyanate trimer. Most preferred blocking group is 3,5-dimethylpyrazole.

The RFL dip may contain an emulsifier. Suitable emulsifiers are selected from emulsifiers comprising rubber latex.

A very suitable pre-dip according to the invention is a composition comprising 2.5 to 4 wt. % of water-dispersed 3,5-dimethylpyrazole blocked isocyanate compound (HDI-trimer), 1 to 2.5 wt. % of 2-vinyl pyridine copolymer (latex) and 90 to 96 wt. % of water (wt. % based on solids).

It is essential that the ratio by weight of blocked isocyanate and latex is between 9 and 0.7. Compositions containing too much or too little latex do not give adhesive properties that are comparable with those of the conventional epoxy pre-dips.

More specifically, the invention thus provides a process for adhering aramid fibers to sulfur-vulcanizable rubbers comprising the steps of (1) immersing fibers in an aqueous dispersion which is comprised of (a) 2.5 to 4 wt. % of water-dispersed 3,5-dimethylpyrazole blocked isocyanate compound (HDI-trimer), (b) 1 to 2.5 wt. % of 2-vinyl pyridine copolymer and (c) 90 to 96 wt. % of water to produce coated fibers; (2) drying and curing the coated fibers to produce pre-dipped fibers; (3) subjecting the pre-dipped fibers to a RFL adhesive dip to produce dipped fibers; (4) drying and curing the dipped fibers to produce dipped fibers; (5) placing the cured dipped fibers in contact with a sulfur-vulcanizable rubber; and (6) curing the vulcanizable rubber (wt. % based on solids).

In another embodiment the invention relates to fibers and cords, wherein the fibers or cords have distributed over at least a surface portion thereof (1) a blocked tri-functional isocyanate, such as water-dispersed 3,5-dimethylpyrazole blocked isocyanate compound (HDI-trimer); (2) a RFL adhesive; and (3) an in situ vulcanized rubber compound.

Also provided is a composite article of rubber and fibers, wherein the fibers have distributed over at least a surface portion thereof (1) a blocked tri-functional isocyanate, such as water-dispersed 3,5-dimethylpyrazole blocked isocyanate compound (HDI-trimer); (2) a RFL adhesive; and (3) an in situ vulcanized rubber compound.

The invention is illustrated by the following non-limitative examples.

EXAMPLE 1

This example provides testing using aramid test sample to show the effect of using a tri-functional blocked isocyanate with 2-vinyl pyridine copolymer together on aramid fibers.

Adhesives

Control adhesive for aramid fiber is a double coat of epoxy and RFL. Aramid fiber was dipped with poly-functional epoxy resin pre-dipping and RFL top-coat for comparison.

Experimental adhesive (sub-coat) consists of Trixene® BI 7986 (3,5-dimethylpyrazole blocked HDI trimer; ex Baxenden Chemicals Ltd., UK) at 3.5 wt. % and Pliocord® VP106S (2-vinyl pyridine copolymer latex; ex Goodyear Chemicals Ltd, USA) at 1.5 wt. % (base on solids) as pre-dipped and RFL as top-coat.

Cord Processing

The aramid cord was prepared with construction of 2 ply yarns. 1680 dtex PPTA yarn (Twaron®, ex Teijin) or 1670 dtex PPODPTA yarn (Technora®, ex Teijin) was twisted as Z direction with 330 tpm. By cabling, two twisted yarns were combined and twisted as S direction with 330 tpm. This greige cord was dipped in pre-dipping adhesive and dried at 150° C. for 120 sec and cured at 240° C. for 90 sec, dipped in RFL, and cured at 235° C. for 90 sec.

Compound for Testing

Dipped aramid cords were tested in the compound for passenger car tire ply rubber. Main component is natural rubber.

Results and Discussions

Evaluation of Trixene® B17986 and Pliocord® VP106S were conducted for aramid adhesives. Dipped aramid cord was also dipped in RFL and tested for static adhesion and the heat durability of adhesion. The results are summarized in Table I.

In this and the following examples, the static peel adhesion and the heat durability of adhesion test refer to a test where the cord is embedded in rubber and the rubber is cured in the metal molding (based on ASTM D 4393; SPAF=Strap Peel Adhesion Force). The composite samples are then subjected to load until the rubber separates from the cord layer, in room temperature atmosphere (static) or 120° C. atmosphere (heat durability), which load is measured in Newton (N). TABLE I PPTA PPODPTA Static Heat Static Heat SPAF in durability SPAF in durability pre-dip¹ N/2 cm (N) N/2 cm (N) control  100/0²  290 (60)³ 280 (45) 200 (30) 210 (35) Comparison 1 100/0 210 (10) 195 (10) 180 (10) 190 (10) Comparison 2 90/10 250 (20) 240 (15) 180 (15) 185 (15) Comparison 3 40/60 260 (30) 230 (25) 175 (10) 180 (15) Test 1 80/20 285 (55) 280 (55) 195 (30) 210 (30) Test 2 70/30 290 (60) 285 (55) 200 (30) 225 (30) Test 3 60/40 285 (60) 280 (50) 195 (35) 220 (35) Test 4 50/50 290 (55) 280 (55) 200 (30) 210 (30) ¹HDI-trimer/latex dip (wt. ratio HDI-trimer/2-vinyl pyridine (Vp) latex) ²epoxide dip instead of HDI-trimer/latex dip ³values between brackets indicate rubber coverage at peeled surface in %

EXAMPLE 2

The adhesion performance of epoxy-free pre-dip according to the invention and epoxy pre-dip according to the prior art were determined by applying standard cord (i.e. Twaron® 1000, 1680 dtex/2, 1Z 330×2S 330) and standard rubber (Dunlop® 5320, NR rubber) and applying the tire carcass rubber (NR/SBR=70/30 wt. ratio).

Yarn and Cord

Twaron® 1000, 1680 dtex/2, 1Z 440×2S 440 (motor cycle tire application) Two types of twisting machine were applied; i.e. two-steps twisting=Rezzeni ring twister, and direct cabling=Saurer Allma)

Dipping

Standard prior art dipping (epoxy pre-dip) (2% of trifunctional glycidyl glycerin ether (epoxide)/RFL)

Dip according to the invention (epoxy-free pre-dip) (non-epoxide/RFL)

HDI-trimer/Vp latex=75/25 wt. ratio, conc. 5%)

Sequence

Pre-dip; 120 s×150 C (9N)—90 s×240 C (9N)

RFL dip; 90 s×235 C (9N)

Evaluation (SPAF)

Standard rubber (Dunlop® 5320; NR) 150 C×20 min

BS/FS rubber (cra) (NR/SBR) 160 C×20 min

Breaking strength of greige and dipped cords are shown in Table II TABLE II Breaking Twisting Breaking strength strength Dip type machine greige (N) dipped (N) epoxide/RFL two-steps 425 423 non-epoxide/RFL* two-steps 425 435 epoxide/RFL direct cabling 410 455 non-epoxide/RFL* direct cabling 410 453 *according to the invention

Adhesion Performance

SPAF performance was determined with NR rubber and NR/SBR rubber for epoxide/RFL (prior art) and non-epoxide/RFL (invention) in both two-steps and direct cabling, and were shown to be almost the same under all conditions. All values were within the range between 280 and 320 N/2 cm.

Rubber Coverage

Rubber coverage was determined as follows. After measuring the peeling adhesion, the peeled surface was observed and the ratio of remained rubber was determined. Results are given in Table III. TABLE III Twisting Dip type machine NR rubber NR/SBR) rubber epoxide/RFL two-steps R—R, R-C R—R 65-70% 100% Non-epoxide/RFL* two-steps R—R, R-C R—R 60-65% 100% epoxide/RFL direct cabling R—R, R-C R—R 60-65% 100% Non-epoxide/RFL* direct cabling R—R, R-C R—R 55-60% 100% NR = natural rubber SBR = styrene-butene rubber R—R = rubber broken R-C = broken between rubber and cord *according to the invention

These results show that epoxy-free pre-dip is comparable with the prior art epoxide dip and is therefore be applicable as a new adhesion method. 

1. A two-step method for dipping a synthetic fiber, comprising dipping the fiber in an aqueous pre-dip composition followed by dipping in a resorcinol-formaldehyde latex (RFL) composition, wherein the aqueous pre-dip composition comprises a blocked isocyanate having at least tri-functional isocyanate groups and a latex and wherein a ratio by weight of blocked isocyanate to latex is in the range of from about 9 to about 0.7.
 2. The method according to claim 1 wherein the pre-dip composition is substantially free from epoxide-containing derivatives.
 3. The method according to claim 1 wherein the latex part of the pre-dip composition and of the RFL composition comprises block copolymers of a conjugated olefinic block and an aromatic block.
 4. The method according to claim 3 wherein the latex part is a vinyl-pyridine block copolymer.
 5. The method according to claim 1 wherein the synthetic fiber is a fiber containing at least one of aramid, polyester, and polyester terephthalate.
 6. The method according to claim 5 wherein the synthetic fiber is a fiber containing at least aramid.
 7. The method according to claim 1 wherein as the synthetic fiber is an as spun fiber.
 8. The method according to claim 1 wherein the blocked isocyanate is a blocked HDI-trimer.
 9. A pre-dip composition for dipping a synthetic fiber, comprising a blocked at least tri-functional isocyanate compound and a latex, further comprising water, an emulsifier, and 2-vinyl pyridine copolymer.
 10. A fiber or cord, wherein the fiber or cord has distributed over at least a surface portion thereof (1) a blocked at least tri-functional isocyanate compound, (2) an RFL adhesives and (3) an in situ vulcanized rubber compound.
 11. A composite article comprising rubber and the fiber or cord according to claim
 10. 12. The method according to claim 1 wherein the ratio by weight of blocked isocyanate to latex is in the range of from about 4 to about
 1. 13. The method according to claim 1 wherein the synthetic fiber is a fiber containing at least PPTA or PPODPTA.
 14. The method according to claim 2 wherein the latex part of the pre-dip composition and of the RFL composition comprises block copolymers of a conjugated olefinic block and an aromatic block.
 15. The method according to claim 14 wherein the latex part is a vinyl-pyridine block copolymer.
 16. The fiber or cord according to claim 10 wherein the blocked at least tri-functional isocyanate compound is a HDI-trimer. 